1. Field of the Invention
This invention relates to a lubricant for a thermoplastic resin and a thermoplastic resin composition comprising thereof.
2. Prior Art
Thermoplastic resins, especially poly(vinyl chloride) resins, are widely used in various fields. In general, a poly(vinyl chloride) resin contains aids such as a lubricant and a processing aid, in order to improve its moldability and processability and to enable it to stably pass through various processes. Polymers such as a copolymer mainly composed of methyl methacrylate have been used as such polymer processing aids or lubricants. For example, JP-B-52-781 discloses a three-stage polymerizing polymer consisting of components of poly(methyl methacrylate)/styrene-(meth)acrylate copolymer/poly(methyl methacrylate); JP-B-52-3668 discloses a two-stage polymerizing polymer consisting of components of poly(methyl methacrylate)/a copolymer of styrene, methacrylate and multifunctional monomer; JP-B-50-37699 discloses a two-stage polymerizing polymer consisting of components of polyacrylate/poly(meth)acrylate, polystyrene or poly(substituted styrene); and JP-A-07-278237 discloses a two-stage polymerizing polymer consisting of components of polyacrylate/methyl methacrylate-vinyl compound copolymer. In addition, JP-A-55-164209 and EP-581009 (U.S. Pat. No. 5,476,906) disclose a copolymer of an acrylate having an alkyl group with at least a particular number of carbon atoms and an acrylate having an alkyl group with below a particular number of carbon atoms.
Neither of the copolymers described above, however, can satisfactorily improve lubricating properties, especially initial lubricating properties, of a lubricant such as delaying melting of a thermoplastic resin, especially poly(vinyl chloride) resin, during processing and its fluidity; reducing a stock pressure required for its molding; releasability from a metal surface of a molding machine; and durability of lubricating properties.
The above processing aids and lubricants are intended to retain transparency of a thermoplastic resin, especially poly(vinyl chloride) resin and improve a long-term durability of lubricating properties such as processability (e.g., drawdown), fluidity during molding, elasticity at an elevated temperature and releasability from a roll surface during calendering, and in terms of these respects they have been successful in some degree.
In the light of improvement of productivity and quality and energy-saving as well as environmental problems, however, conventional lead-containing processing aids which have good lubricant properties have been recently being replaced with non-toxic ones. Furthermore, there exist needs for lubricants having features such as good fluidity during molding of a thermoplastic resin, especially poly(vinyl chloride) resin, a reduced stock pressure required for molding, good initial lubricating properties and its durability, providing a molded product with a good luster and transparency. Given such a situation, the above processing aids and lubricants do not satisfactorily meet these needs. In addition, lubricants should have excellent powder properties, especially untiblocking, which neither of the above materials meet.